Method and apparatus for measuring the degree of polarization of polarized gas

ABSTRACT

The invention relates to a method for determining the degree of polarization (P) of a nuclear spin polarized gas, in particular  3 He,  129 Xe, in which the nuclear spin polarized gas is placed in a container, comprising determining the magnetic field B d  of the pola gas by measuring the magnetic dipole field emerging therefrom and then determining from B d  the degree of polarization of the gas.

METHOD AND APPARATUS

The invention relates to a method and apparatus for determining the degree of polarisation of nuclear spin polarised gases, in particular 3He or ¹²⁹Xe.

Nuclear spin polarised gases, such as the helium isotope with mass number 3 (³He) or the isotope of xenon with mass number 129 (¹²⁹Xe) and gases containing the fluorine, carbon or phosphorus isotopes ¹⁹F, ¹³C or ³¹P, are required for a wide variety of basic physics research experiments.

In the medical field, such isotopes have particularly been discussed for use in nuclear spin tomography (magnetic resonance imaging), for example of the lung. (See for example WO95/27438, WO97/37239, Bachert et al., Mag. Res. Med. 36: 192-196 (1996) and Ebert et al., The Lancet 347: 1297-1299 (1996)). In addition, Noël et al., J. Phys. III France 6: 1127-1132 (1996) discloses a ⁴He magnetometer used to detect the static magnetic field produced by optically pumped ³He nuclei submitted to RF discharge. Similarly, Cohen-Tannoudji et al., Phys. Rev. Letts. 22: 758-760 (1969) discloses the use of a sensitive low-field magnetometer to detect the static magnetic field produced by optically pumped ³He nuclei in a vapor. To be useful in nuclear spin tomography, the nuclear spin polarised gases require a degree of polarisation P of spin I of the atomic nucleus, or the nuclear magnetic dipole moment μ_(I) connected therewith, which is about 4-5 orders of magnitude larger than P_(Boltzman) the degree of polarisation of the gas in its relaxed state in normal thermal equilibrium in the magnetic field B_(T) of the mr imaging apparatus. P_(Boltzmann) is related to the Boltzmann constant, the magnetic dipole energy −μ_(I)B_(T) and thermal energy kT by:

 P _(Boltzmann)=tan h(μ _(I) B _(T) /kT)  (1)

(where k=Boltzmann constant, and T=absolute temperature in Kelvin).

Where P_(Boltzmann) <<1, then it approximates to μ_(I) B_(T)/kT.

Since routinely B_(T)=1.5T and T=300K for the hydrogen isotope ¹H used in tissue tomography, it has a P_(Boltzmann) of only 5×10⁻⁶, but in gas tomography a P>1×10⁻² (i.e. 1%) is required. The requirement for such an extremely high P is mainly due to the low concentration of gas atoms in comparison to that of hydrogen in tissue. Gases with such degrees of polarisation (normally referred to as “hyperpolarised gases”) can be prepared using various known methods, advantageously by optical pumping or by polarization transfer.

In addition, large amounts of hyperpolarised gas, for example of the size of an intake of breath (0.5 to 1 litre) must be prepared for use.

Particularly high degrees of polarisation—for example >30%—in simultaneously high production amounts, for example 0.5 liters/h, may be achieved by compression of an optically pumped gas. This method is described in the following publications:

Eckert et al., Nuclear Instruments and Methods in Physics Research A 320: 53-65 (1992);

Becker et al., J. Neutron Research 5: 1-10 (1996);

Surkau et al., Nuclear Instruments and Methods in Physics Research A 384: 444-450 (1997); and

Heil et al., Physics Letters A 201: 337-343 (1995).

The extremely costly production of hyperpolarised gases, for example using the methods described above, generally involves production at a site remote from the place of use. As a result, they must be transported from the place of production to the user. Since a wide variety of relaxation processes (e.g. wall relaxation, relaxation in inhomogeneous, external, stray magnetic fields, etc.) causes the gas to depolarise to a greater or lesser extent, it is necessary to determine the degree of polarisation before using the hyperpolarised gas, for example in medical imaging.

One problem is that this must be determined as precisely as possible despite stray fields or applied fields. Further, the determination should be performable by relatively inexperienced personnel, ie. personnel who are not experts in the physics of hyperpolarized gases.

The present invention solves the above problem by providing a method for determining the degree of polarisation of nuclear spin polarised gases by exploiting the fact that nuclear spin polarisation of gases produces magnetic fields B_(d) in the nanoTesla to microTesla (nT to μT) range.

Thus viewed from one aspect the invention provides a method of determining the degree of polarisation (P) of a nuclear spin polarised gas in a container, said method comprising determining the magnetic field B_(d) of the polarised gas using a magnetic field sensor and then determining therefrom the degree of polarisation of the gas.

In the method of the invention, the shape and size of the container into which the polarised gas is placed is important. Thus, the magnetic field B_(d), which is dependent on the degree of polarisation of the gases, may be determined using a magnetic field sensor, e.g. a magnetometer, rather than a nuclear magnetic resonance polarimeter as has been used in the past, and the absolute degree of polarisation can be determined from B_(d) by taking into consideration the geometric shape of the container in which the gas is placed, the type of gas and its density, and the arrangement of the sensor relative thereto.

If, as is preferred, the container in which the gas is received is spherical in shape, then the magnetic field has a field gradient like that formed by a point dipole.

Thus for a spherical container, the magnetic field B_(d) of the polarised gas on the equitorial outer surface of the container deriving from the orientated nuclear magnetic dipole moment of the nuclear spin gas is: $\begin{matrix} {B_{d} = {{- P} \cdot n \cdot \frac{R^{3}\mu_{0}}{3r^{3}} \cdot \mu_{N}}} & (2) \end{matrix}$

where P represents the degree of polarisation to be determined and n the particle density of the gas. The factor R³/3r³ is termed the geometry factor, and depends on the shape of the container and thus on the volume in which the nuclear spin polarised gas is dispersed. R represents the radius of the sphere and r the distance of the sensor from the centre point of the container sphere perpendicular to the dipole axis. μ₀=1.257×10⁻⁶ Vs/Am, i.e. the permeability of vacuum, and μ_(N)=1.075×10⁻²⁶ Am², the nuclear dipole moment of the gas (in this case ³He)

Similar equations to equation (2) may be generated for containers which are non-spherical.

The geometric factor also takes the position of the magnetic field measuring apparatus relative to the direction of the magnetic field of the gas into consideration. If the field emerges from the poles of the container, the sensor is positioned in the equatorial plane of the spherical gas container.

Different geometric factors must be used for different container geometries, as in each case there is a different calculable field gradient of magnetic field B_(d). If the geometric factor, the distance from the measuring sensor and the particle density of the-nuclear spin polarised gas in the container are known, then equation (2) can be used to determine the absolute degree of polarisation P from the B_(d) determined using the measuring apparatus.

As an example, assuming a degree of polarisation P=50% and a particle density n=10²⁰/cm³, then the field at the edge of the sphere (r=R) has a value B_(d)=0.22 μT. This value is of the order of 1 thousandth of the homogeneous magnetic field caused by the polarisation, similar to, for example, transport fields of 0.3 mT, for example, or external stray fields.

In a preferred implementation of the invention, it is proposed that the sensor comprises a very sensitive magnetic field sensor. In this respect, SQUIDs or more preferably sensors operating on the Forster principle can be considered. Förster sensors operate on the principle of saturable-core magnetometers. The measuring element of saturable-core magnetometers essentially consist of one or more narrow cores of highly permeable materials (μ-metal or ferrite) with almost linear B(H) behaviour.

In a variation of a saturable-core magnetometer, a double core sensor comprises two mutually parallel cores each provided with a primary and a secondary winding. The former are opposed, the latter are connected in series. The primary winding is supplied with a constant current by means of a low frequency transmitter (ν=50 to 1000 Hz). The current intensity is sufficient to magnetise the highly permeable cores to saturation. A voltage is induced in the secondary windings by the changing magnetic field of the primary coils.

With no external magnetic field, the primary fields in the two cores are equal and opposite. Similarly, in the two secondary coils during the time in which the magnetic flux density B changes, equal and opposite voltages are induced, which add up to zero. In the presence of an external field component H₀ parallel to the longitudinal axis of the cores, this symmetry shifts by the value of H₀. The operating point on the B(H) curve is shifted, and the induced impulses no longer add up to zero, since with a change of H in one core, saturation is achieved faster than in the other. The result is that the voltage pulses of the derivative with time of the flux density dB/dt appear in the two cores at different times. The sum dB₁/dt+dB₂/dt goes from zero to different signals, the breadth and distance apart in time of which are dependent on the amplitude of the external magnetic field H₀ and serve to determine the size of H₀.

An example of a commercial sensor which operates using the principle described above is the MAG-03 MS-sensor from Bartington Instruments Ltd.

With such magnetic field sensors, an accuracy of about 5 nT can be achieved in the measurement range B<1 mT. Thus it is possible to determine the magnetic field B_(d) with such sensors.

In order to be able to determine the polarisation- dependent magnetic field B_(d) of polarised gas in the presence of an applied field B₀ (e.g. the ambient field or a substantially uniform generated magnetic field within a transporter device), the polarisation-dependent magnetic field is advantageously determined by displacing the measuring apparatus and container (gas storage cell) relative to each other. This is advantageously achieved in that in a first position as close as possible to the wall of the container containing the nuclear spin polarised gas, a magnetic field sensor is used to record the field value, constituted by the field of the applied field B₀ and the field of the nuclear spin polarised gases in the container (B_(d) at the equatorial plane of the container). After recording this signal the container is moved relative to the sensor to a position distanced from the sensor, preferably in the direction of the axial-applied field, by at least five times the radius of the container. The field component caused by the nuclear spin polarised gas then falls to less than 1% of its original value. This means than in this position only the value of the applied field B₀ is measured. The difference between these two signals can be used to determine the value of B_(d) and thus the degree of polarisation P can be determined using equation (2) given above. It is clearly possible to increase the accuracy of the results obtained for the degree of polarisation using this method by making a series of measurements. In a further embodiment, the magnetic field sensor is displaced from the container. If the applied field B₀ changes on displacing the sensor, this must be taken into consideration when calculating the degree of polarization. Moreover if measurement is not made in the equatorial plane of the container, this too must be taken into consideration. Thus if the field is measured in the polar direction of the container, the difference is B_(d)′=2×B_(d).

These two methods have the advantage that commercially available magnetic field sensors can be used even by inexperienced personnel to determine the magnetic field of the nuclear spin polarised gases to an accuracy of 10%, advantageously 2%. With regard to geometric uncertainties, polarisation determination to 50%, advantageously to <10%, is quite possible.

In a second embodiment of the method of the invention B_(d), and hence P, may be determined by using a high frequency magnetic pulse to reverse the polarisation and by measuring the resulting magnetic field change ΔB without moving the sensor and the container (the gas storage cell) relative to each other.

In this method, the polarisation-dependent magnetic field is advantageously determined by applying a high frequency magnetic pulse over the applied field, so that the sign of P is reversed by nuclear magnetic resonance. To this end, suitable coils or a solenoid are used to emit a high frequency magnetic field pulse of varying amplitude and frequency

B(t)=B ₁(t).cos(ω(t).t)  (3)

perpendicular to the applied field B₀.

This magnetic field change based on the sign reversal of P preferably occurs on the principle of “fast adiabatic passage”, fully described in A. Abragam, “The Principles of Nuclear Magnetism”, Oxford University Press, London, England, 1973 (see especially pages 34-36 and 65-66). In this method, the frequency ω(t) of the high frequency magnetic field pulse during the pulse period is pushed beyond the resonance frequency of the nuclear dipole moment: $\begin{matrix} {\omega_{0} = {\frac{2{\pi\mu}_{N}}{h\quad I_{N}} \cdot B_{0}}} & (4) \end{matrix}$

where h is Planck's constant, and I_(N) is the nuclear spin quantum number. If the pulse period is short and the high frequency field strength B_(d) (t) is chosen correctly, the polarisation is completely reversed with no reduction in magnitude. This means that the reading of the magnetic field sensor changes by an amount

ΔB=2B _(d)  (5)

In comparison with the method described above, in which the sensor and container are displaced with respect to each other, the method in which the field B_(d) is determined using a magnetic field pulse has the advantage that a measuring signal ΔB=2B_(d) which is twice as large is obtained.

A complete reversal is obtained using the principle of “fast adiabatic passage” (see Abragam (supra)) if the following conditions are satisfied as regards emitting a high frequency pulse from a magnetic field pulse transmitter:

1. The applied high frequency magnetic field strength B₁ must be large in comparison with the magnetic field variation ΔB₀ which the applied field B₀ exhibits because of inhomogeneities in the container dimensions.

2. The frequency shift Δω between the start and end of the high frequency pulse must be large compared with the broadening of the nuclear resonance lines caused by the field variation ΔB₀.

3. The pulse duration Δt must be short compared with the characteristic transverse relaxation time T₂of the gas.

4. The product B₁. Δt must be large compared with hI_(N)/(2Πμ_(N)).

This polarisation reversal method is particularly suitable as a complete polarisation reversal can also be achieved in an extensive gas volume, although the applied field B₀ for maintaining the polarisation can vary slightly spatially by ΔB₀. The method is thus robust and guarantees reproducible results, even when the nuclear spin polarised gas container is changed or, for example, external stray fields are superimposed, as may be the case in different locations. The method can advantageously be used in transportable magnetic fields to determine the polarisation of a gas on site even by inexperienced personnel.

A further method of producing a polarisation reversal is to apply a magnetic pulse using the principle of the 180° nuclear resonance pulse or “Πpulse”, as fully described in A. Abragam (supra) pages 32-34.

In this method, suitable coils or a solenoid are used to produce a high frequency magnetic field pulse of varying amplitude and frequency (equation (3)) perpendicular to the applied field.

For a complete reversal of the nuclear spin polarisation of the gas using the n pulse principle, a magnetic field pulse with frequency ω₀ (equation (4)) must be applied by a magnetic field pulse transmitter using the following conditions:

1. The applied high frequency magnetic field strength B₁ must be large in comparison with the magnetic field variation ΔB₀ which the applied field B₀ exhibits because of inhomogeneities in the container dimensions.

2. The pulse duration Δt must be short compared with the characteristic transverse relaxation time T₂ of the gas.

3. The relationship $\begin{matrix} {{j \cdot \pi} = {{\frac{2{\pi\mu}_{N}}{h\quad I_{N}} \cdot B_{1} \cdot \Delta}\quad t}} & (6) \end{matrix}$

must be satisfied, where h is Planck's constant and I_(N) is the nuclear spin quantum number, μ_(N) is the nuclear dipole moment of the isotope under consideration, and j=1,3,5,7, etc, however j=1 is normally selected.

In contrast to the “fast adiabatic passage” method, in the n pulse method the relationship (6) must hold exactly. This is rendered more difficult if variations ΔB₀ occur in the applied field B₀ over the gas volume.

For larger gas volumes, as considered in the present invention, significant field variations ΔB₀ occur over the gas volume so the more robust “fast adiabatic passage” method with the particular advantage of complete reversal of the nuclear spin polarisation of the gas is preferred. This does not, however, constitute a limitation of the inventive concept of measuring a polarisation reversal with a magnetic measuring apparatus using nuclear resonance methods.

Viewed from a further aspect, the present invention also provides apparatus for determining the magnetic field (B_(d)) of a nuclear spin polarised gas (and preferably also for determining the degree of polarisation P thereof), said apparatus comprising a magnetic field sensor arranged to determine the magnetic field at at least two positions relative to a container containing a nuclear spin polarised gas, optionally magnetic field applying means arranged to apply a magnetic field to said container, and optionally computing means for determining the degree of polarisation of the polarised gas from the magnetic fields determined by the sensor.

In this apparatus, the sensor may operate to determine the magnetic fields at the various relative positions or it may simply determine the field difference between the positions.

In the apparatus, the sensor may be movable relative to the container or alternatively it may comprise separate sensors located at different positions relative to the container. The different positions will generally include positions relative closer to and further away from the container. The relative motion of sensor and container may be achieved by moving sensor and/or container to preset receiving sites or along a guide, e.g. using a drive means (for example a motor-driven or hand operated drive means)

In this apparatus, the means for applying a magnetic field are preferably means, e.g. a permanent or electromagnet, for applying a substantially uniform field B_(o).

Viewed from an alternative aspect the invention also provides apparatus for determining the magnetic field (B_(d)) of a nuclear spin polarised gas (and preferably also for determining the degree of polarisation (P) of the gas), said apparatus comprising a magnetic field sensor and means for applying a time variant magnetic field to a container containing a nuclear spin polarised gas, optionally also means for applying a substantially uniform magnetic field to said container, and optionally computing means for determining the degree of polarisation of the polarised gas from the magnetic field variation determined by the sensor.

In this second form of apparatus according to the invention, there may if desired be two or more sensors and the sensor and container may be movable relative to each other.

In both forms of the apparatus of the invention the container is preferably spherical and the sensors are preferably highly sensitive, e.g. SQUIDs or Forster principle magnetometers.

The first apparatus of the invention thus conveniently comprises apparatus for determining the degree of polarisation (P) of nuclear spin polarised gases in which the nuclear spin polarised gas is placed in a container, the apparatus comprising: at least one highly sensitive magnetic field sensor, wherein the sensor and the container are arranged so as to be displaceable relative to each other, so that the magnetic field can be determined in at least two locations, e.g. one close to and one distanced from the container, and thus the magnetic field B_(d) can be determined.

The second apparatus of the invention also conveniently comprises apparatus for determining the degree of polarisation (P) of nuclear spin polarised gases in which the nuclear spin polarised gas is placed in a container, the apparatus comprising: at least one highly sensitive magnetic field sensor, and a high frequency magnetic field pulse transmitter arranged to emit a high frequency magnetic field pulse of variable amplitude and frequency.

The apparatus of the invention is desirably incorporated into apparatus for transporting hyperpolarised gases in which the container is placed within an area of highly uniform applied magnetic field in a chamber within the transporter apparatus.

In a special embodiment, the high frequency magnetic field pulse transmitter comprises coils or solenoids. In one particular embodiment, the magnetic field pulse transmitter is constructed so that a magnetic field pulse is emitted by which the polarisation is completely reversed with no reduction in magnitude.

The use of the apparatus and method of the invention are particularly advantageous with respect to the prior art for the following reasons:

Commercially available apparatus components (e.g. sensors) with high precision and with reproducible results with a relative error of only 0.5% or less can be used. These commercially available apparatus components are calibrated so expensive calibration can be avoided.

The prior art methods for determining the nuclear spin polarisation which induce small nuclear resonance excitations are based on recording the dynamic nuclear spin resonance signal thus produced, recorded with a receiver apparatus. A very costly calibration is necessary for absolute polarisation determination, in which very small resonance signals are measured, which are very sensitive to disturbances from external influences such as container geometry or the construction of the receiver apparatus. In order to produce good results, each receiver apparatus must be separately calibrated, and that calibration is only valid for one container geometry. Using a magnetic field sensor as in the invention means that the very expensive calibration of the receiver apparatus and standardisation of the container geometry is no longer required. The influence of container geometry can be readily calculated. The static magnetic field of the nuclear spin polarised gas is measured. Then a series of measurements of the static magnetic field before and after polarisation reversal can be made to substantially increase the accuracy of the method. The measuring method can thus advantageously be embodied in a measuring apparatus. The measuring apparatus itself is highly reproducible and reliable. In particular, the measuring method is also suitable for use by inexperienced personnel because of its robustness.

Embodiments of the invention will now be described with reference to the accompanying drawings, which are provided by way of illustration and are in no way limiting and in which:

FIG. 1: shows a perspective external view of a transport apparatus for hyperpolarised gas;

FIG. 2: shows a cross section through a transport apparatus comprising a magnet (a pot magnet), and a storage cell for nuclear spin polarised gases arranged inside;

FIG. 3: shows a section through a further embodiment of the apparatus of the invention;

FIG. 4: shows the determined magnetic field using a first apparatus according to the invention in which the container is placed on the magnetic field sensor or distanced from the sensor; and

FIG. 5: shows the determined magnetic field using a second apparatus according to the invention using the “fast adiabatic passage” method.

Referring to FIG. 1 there is shown a perspective external view of an embodiment of a transport apparatus, formed as a two-part cylinder-shaped pot magnet 1 with an upper portion 1.1 and a lower portion 1.2. The Figure also shows the axis of rotational symmetry S of the pot magnet and the magnetic field lines of external magnetic fields, for example the earth's magnetic field. It particularly shows the transverse component of an external magnetic field or stray field B_(s) ^(⊥), which does not penetrate inside the pot magnet, but because of the small magnetic resistance of the yoke 2 (which is preferably formed from soft iron) is directed around the internal space. The stray field component B_(s) ^(II), which is parallel to internal field B_(o), is perpendicular to the yoke base and is homogenised by μ-metal or soft iron pole pieces located internally of yoke 2 and hence contributes to B₀ without affecting its homogeneity.

FIG. 2 shows an axial cross section through the transport apparatus of FIG. 1, useful in particular for transporting nuclear spin polarised ³He or ¹²⁹Xe, especially ³He. The apparatus comprises a pot magnet apparatus with a container for the nuclear spin polarised gas located within it. Using this transport apparatus the gas exhibits an extremely long wall depolarisation relaxation time.

The pot magnet 1 comprises a box-shaped yoke 2, preferably formed from soft iron to repel the magnetic flux and to protect from external fields. The box-shaped yoke 2 also comprises two yoke bases as middle portion 2.1. The yoke bases 2.1 in this case may for example comprise two circular (or polygonal) disks 2.1.1 and 2.1.2. At the edge of the yoke bases, circumferential closed strips 2.2 and 2.3 are arranged to form a casing. These are different in the embodiments shown in the left and right halves of FIG. 2. The circumferential strips 2.2 or 2.3 are arranged on both the upper disk 2.1.1 and the lower disk 2.1.2, giving the pot magnet an upper and a lower portion, which in the embodiment illustrated on the left press together in the middle plane of the magnet apparatus at the bent flanges 2.2.1. In the second embodiment shown on the right, the flanges 2.3.1 are at a distance apart, to form an opening for field sources 2.4, for example permanent magnets, in the middle plane of pot magnet 1. The field line gradient resulting from positioning the field source, for example a permanent magnet, in the middle between the upper and lower edge of the pot magnet is shown at 6. In the first embodiment shown on the left, the height of the two yoke casing halves 2.2 is greater than the distance apart of yoke bases 2.1.1, 2.1.2. In the gap between casing and base, it is possible to arrange field sources in a facing position 2.5. The field line gradient at the edge region resulting from this arrangement is shown at 8.

The two opposed pole shoes 10.1 and 10.2 are responsible for the homogeneous field within the pot magnet. The pole shoes are formed from parallel, opposed, magnetic field homogenising plates, e.g. of μ-metal or soft iron in the embodiment shown. μ-metal is a material with a very high homogenising power for an external magnetic field, for example a stray magnetic field B_(s) ^(II), and is characterized by a very small remanence.

In the present embodiment, μ-metal A from Vacuumschmelze, Postfach 2253, 63412 Hanau, Germany, was used which had the following properties:

Stat. Coercive field strength H_(c) ≦30 mA/cm Permeability μ₍₄₎ ≧30000 Maximum permeability μ_(max) ≧70000 Saturation induction B_(S) ≧0.65 T

Other materials however can be used.

The pole shoes are desirably kept in opposed parallel relationship by three or more spacers in or by circular or polygonal spacer rings. As illustrated in FIG. 2, the distance between the μ-metal plates is ensured by three spacers 12, only one of which is shown.

The resulting homogeneous magnetic field between the pole shoes 10.1 and 10.2 of μ-metal is shown at 14 in the present embodiment. As can be seen from the embodiment shown in FIG. 1, a particularly homogeneous magnetic field is produced inside the pot magnet due to the homogenising power of the μ-metal, independent of external fields, while in the edge regions, depending on the arrangement of the field sources, a deviating field gradient 6 or 8 is produced. If the field sources are only arranged in the middle plane 4, as shown in the right hand side of pot magnet 1, a substantial portion of the magnetic flux extends outside the casing because of the small magnetic resistance and passes through more strongly from the edge into the field between the pole shoes. The field thus falls substantially from the edge inwards and the desired homogeneity is destroyed even at a relatively small distance between the two pole pieces. By arranging the permanent magnets in a facing position on the pot bases, as shown in FIG. 2 for the left hand region of the magnet, the field drops off substantially between the pole shoes 10.1, 10.2 as shown by field line 8, because the casing close to the pole shoes pulls on the edge field and weakens it.

The very homogeneous field 14, produced in the space between the pole shoes because of the extremely high permeability of the μ-metal or soft iron plates used as pole shoes 10.1, 10.2, can be further strengthened by inserting a magnetic resistor 16 between pole shoes 10.1, 10.2, and the yoke 2.1.1 or 2.1.2. Preferably, a deformation-resistant, non-magnetic plate, for example a plastic plate 16, or, to save weight, a honeycomb structure, is used. Plate 16 can be glued to pole shoes 10.1, 10.2 and thus ensures that the pole shoes 10.1, 10.2 are flat and parallel.

The container (storage cell) 20 for the polarised gas is in the central middle portion of the magnetic field containing chamber 26 in pot magnet 1 between the two pole shoes 10,1, 10.2. Container 20 is preferably formed from iron-free glass and has, for example, an iron concentration of less than 20 ppm. Advantageously, the container is so constructed that it also constitutes a high barrier to helium diffusion. In this way, wall relaxation times of more than 70 hours are achieved. Storage cell 20 can be pumped out before use and as is normal in high vacuum technology, it can be heated to remove the remaining layers of water. This measure is of advantage in the present invention but not necessary. The storage cells are closed, for example, using a normal glass tap 22 and are connected to a filling point for polarised gas via a glass flange 24.

In order to determine the degree of polarisation, in accordance with the invention, an apparatus for determining the degree of polarisation is provided inside the transport apparatus.

In a first embodiment of the invention, only one magnetic field sensor 32 is arranged in the transport container, e.g. a sensor which operates using the Förster principle and, for example, obtains its measuring signal by saturation of a highly permeable transformer core by the external field. A commercially available example of such a magnetic field sensor is the MAG-03 MS sensor from Bartington Instruments Ltd, with which a measuring range B<1 mT can be obtained, for example with an accuracy of about 5 nT.

When the magnetic field B_(d) produced by nuclear spin polarisation is to be determined using such an arrangement, then the magnetic field is first determined using the magnetic field sensor initially in position A as shown near to the cell. In this position, the magnetic field is constituted by the value of the applied field B₀ plus the value of the nuclear field B_(d) formed by the nuclear spin polarisation. The cell must then be displaced relative to the magnetic field sensor. This can be effected by moving the cell in the direction of the axial applied field by at least five times its radius or by moving the magnetic field sensor by this value. Once in position, the field at such a distance from the storage cell for the polarised gas will only reflect the applied field B_(o). The difference between the two values enables B_(d) to be determined using equation (2).

In order to further increase the accuracy of the measurements, a further embodiment can be used, in which in addition to the magnetic field sensor a magnetic field pulse transmitter is provided, for example that shown in FIG. 2 as a HF-coil pair 30, with which a high frequency magnetic field pulse of varying amplitude and frequency (Equation (3)) perpendicular to the applied field B₀ can be produced. By appropriate choice of the high frequency magnetic field pulse, it is possible using the “fast adiabatic passage” method to produce a complete reversal of the nuclear spin polarisation in an external applied field. The static dipole field before (B₀+B_(d)) and after (B₀−B_(d)) the polarisation reversal is then measured using the highly accurate magnetic field sensor 32. In this way, a signal 2B_(d) is obtained to give the dipole field produced by the nuclear spin polarisation directly from such a measurement.

The conditions for adiabatic fast passage require a relatively homogeneous magnetic field such as the applied field. The applied field B₀ described for the transport apparatus satisfies these conditions.

FIG. 3 shows an alternative embodiment of the invention, in which the magnetic field pulse transmitter uses a solenoid 300 instead of HF coils.

FIG. 4 shows the magnetic field measured as a function of time for a container of nuclear spin polarised ³He gas obtained by placing the container on the magnetic field sensor and then distancing it from the sensor. In this example, a spherical container with a diameter of 5.5 cm was filled with 2.68 bar of ³He (at 295 Kelvin) and displaced in an apparatus such as that of FIG. 2. A′ is the measured field adjacent the container and B′ is the field measured remote from the container, ie. essentially the field B_(o). The step-shaped trace clearly shows the applied field B₀ of 449.805 μT adding to the hyperpolarised magnetic field B_(d)′ of 27 nT.

The field measured in non-limiting fashion in the pole position of the container of B_(d)′=2.B_(d). This enabled a polarisation of 29% to be calculated. This example is solely by way of illustration and in no way limits the inventive concept.

FIG. 5 shows the magnetic field measured as a function of time for a container of hyperpolarised ³He using polarisation reversal by “fast adiabatic passage”. It shows the voltage signal of the magnetic field measuring apparatus. At time t=8 s, polarisation reversal occurs, as is clearly shown by the jump in the points during a series of measurements. Following the development with time of the external magnetic field at the sensor location before and after the polarisation reversal advantageously enables systematic variations which are independent of the nuclear spin polarisation of the gas to be detected. Such variations can, for example, be due to changes in the earth's magnetic field, perhaps by moving magnetic materials. While dynamic nuclear resonance signals can be perturbed by such, the determination of static magnetic fields of nuclear spin polarised gas using a series of measurements is robust.

The present apparatus and method allow a measurement of nuclear spin polarisation which requires no costly calibration and does not use a dynamic nuclear resonance signal to determine B_(d), but directly measures the static dipole field of the polarised gas. Problematic calibration of dynamic nuclear resonance signals can thus be avoided. The methods presented here for measuring the degree of polarisation, in particular using the fast adiabatic passage method, enable- even a non skilled person to readily determine the degree of nuclear spin polarisation. 

What is claimed is:
 1. A method of determining the degree of polarisation of a nuclear spin polarised gas in a container comprising displacing said container and a magnetic field sensor relative to each other, measuring said magnetic field before and after movement of the container and the magnetic field sensor to obtain measurable magnetic field signals, detecting the magnetic field B_(d) as a function of the difference between the measurable magnetic field signals, and determining the degree of polarisation of the gases from the detected magnetic field B_(d).
 2. A method of determining the degree of polarisation of a nuclear spin polarised gas in a container, comprising applying a high frequency magnetic field pulse to the gas to reverse the polarisation of the gas, measuring the magnetic field before and after application of the high-frequency magnetic field pulse to obtain measurable magnetic field signals, detecting the magnetic field B_(d) such that the difference between the measurable magnetic field signals is twice the value of the magnetic field B_(d), and determining the degree of polarisation of the polarised gas from the detected measurable magnetic field B_(d).
 3. The method as claimed in claim 1 wherein the calculation of the degree of polarisation of the polarised gas is dependent on the density of the polarised gas and a geometry factor dependent on the shape and size of the container.
 4. The method as claimed in claim 2 wherein the calculation of the degree of polarisation of the polarised gas is dependent on the density of the polarised gas and a geometry factor dependent on the shape and size of the container.
 5. The method as claimed in claim 1 wherein the container is spherical.
 6. The method as claimed in claim 2 wherein the container is spherical.
 7. The method as claimed in claim 1 wherein the polarized gas is selected from the group consisting of ³He, ¹²⁹Xe and polarised gases containing ¹⁹F, ¹³C or ³¹P.
 8. The method as claimed in claim 2 wherein the polarized gas is selected from the group consisting of ³He, ¹²⁹Xe and polarised gases containing ¹⁹F, ¹³C or ³¹p.
 9. The method as claimed in claim 1 wherein the magnetic field sensor has a measurement accuracy of 5 nT and is capable of determining B_(d) with an accuracy of within 50%.
 10. The method as claimed in claim 1 wherein the magnetic field sensor has a measurement accuracy of 5 nT and is capable of determining the magnetic field B_(d) with an accuracy of within 50%.
 11. The method as claimed in claim 1 wherein the sensor has a measurement accuracy of 5nT and is capable of determining the magnetic field B_(d) with an accuracy of within 10%.
 12. The method as claimed in claim 2 wherein the magnetic field sensor has a measurement accuracy of 5 nT and is capable of determining the magnetic field B_(d) with an accuracy of within 10%.
 13. The method as claimed in claim 1 wherein the container is placed in a substantially uniform applied magnetic field B_(o).
 14. The method as claimed in claim 2 wherein the container is placed in a substantially uniform applied magnetic field B_(o).
 15. The method as claimed in claim 1 wherein said container and said magnetic field sensor are within a magnetized gas transport apparatus.
 16. The method as claimed in claim 2 wherein said container and said magnetic field sensor are within a magnetized gas transport apparatus.
 17. The method as claimed in claim 2 wherein the polarisation reversal is obtained by a fast adiabatic passage.
 18. The method as claimed in claim 2 wherein the high frequency magnetic field pulse is a 180° or Π nuclear resonance pulse.
 19. Apparatus for determining the degree of polarisation of a nuclear spin polarised gas comprising a magnetic field sensor and a container containing a nuclear spin polarised gas, said magnetic field sensor and container being displaceable relative to each other so that the magnetic field can be determined in at least two locations, and magnetic field applying means arranged to apply a magnetic field to the container.
 20. The apparatus of claim 19 further comprising computing means to determine the degree of polarisation of the polarised gas from the magnetic fields determined for the at least two locations by the magnetic field sensor.
 21. Apparatus for determining the degree of polarisation of a nuclear spin polarised gas comprising a magnetic field sensor a container containing a nuclear spin polarised gas, and a high frequency magnetic field pulse transmitter arranged to apply a time variant, magnetic field to said container which reverses the polarisation of said gas.
 22. The apparatus of claim 21 further comprising means for applying a substantially uniform magnetic field to said container.
 23. The apparatus of claim 21 further comprising means for determining the degree of polarisation of the polarised gas from the time variant magnetic field applied to the container as measured by the magnetic field sensor.
 24. The apparatus as claimed in claim 23 wherein the high frequency magnetic pulse transmitter comprises at least one coil.
 25. The apparatus as claimed in claim 21 wherein said container is spherical.
 26. The apparatus as claimed in claim 21 in the form of a transport apparatus wherein said container and magnetic field sensor are enclosed in a magnetic field chamber. 